Modulation techniques in which an external electric field is utilized to move a liquid have been studied in the field of optical devices such as image display devices, optical shutters, optical pickup devices, and liquid optical lenses. Representative examples of such modulation techniques include an electroosmosis technique, an electrophoretic technique, an electrofluidic technique, and an electrowetting technique.
Among these, an electrowetting technique enables a high contrast ratio and a wide viewing angle and precludes use of a front light and back light; hence, use of the electrowetting technique in image display devices which operate with low power consumption has been studied. The principle thereof is based on the phenomenon called as “electrocapillary” as disclosed in Patent Literatures 1 and 2; in particular two separated liquids, namely, two immiscible liquids, which have different color tones and degrees of transparency are used, and the size of a droplet of one liquid put in the other liquid is changed on the basis of application or non-application of voltage, thereby forming colored images. It is possible, for example, to change the size of a droplet of a colored liquid, such as black liquid, put in a colorless liquid to form a color image, such as a black image, on a transparent background; to change the size of a droplet of a colorless liquid put in a colored liquid to form a transparent image on a colored background; and to change the size of a blue droplet put in a red liquid to form a purple image on a red background.
Since such two separated liquids, namely, two immiscible liquids, which have different color tones and degrees of transparency (hereinafter referred to as “separated colored liquid composition”) need to be in a separated state, in other words, an immiscible state, nonpolar solvents such as silicone oil and polar solvents such as water, alcohol, and ethylene glycol are generally used, and a colorant is added to at least any one of these solvents (a liquid to which a colorant has been added is hereinafter referred to as “colored liquid”, and the other liquid is hereinafter referred to as “liquid immiscible with the colored liquid).
In an example in which a colorant is added to a polar solvent, Patent Literature 3 discloses a technique involving use of a colored liquid in which an ionic liquid containing an ambient temperature molten salt produced by a combination of a cation and an anion and a self-dispersible pigment having a functional group, such as a carboxyl group, a hydroxyl group, a carbonyl group, a sulfone group, a hydroxyl group, and a phosphate group, have been added to a polar solvent. Furthermore, Patent Literature 4 discloses a technique involving use of a colored liquid in which a pigment or a dye has been added to a polar solvent having a specific viscosity and surface tension and which has a specific electric conductivity and ionic radius.
In an example in which a colorant is added to a nonpolar solvent, Patent Literature 5 discloses a technique involving use of a colored liquid in which an organic pigment and/or an inorganic pigment, a solvent-soluble or solvent-dispersible polymer dispersant, and an aldehyde resin or a ketone resin have been added to a nonpolar solvent such as decane, decalin, or tetralin.
In image display devices in which such a separated colored liquid composition is used, liquids having different polarities contact each other at the interface therebetween. Hence, in the case where a colorant contained in the colored liquid intrudes into the liquid immiscible with the colored liquid across the interface, the switching performance in optical modulation brought about by a change in the size of a droplet of the colored liquid by application of voltage is reduced or lost. Thus, the demand characteristic of the separated colored liquid composition for long-term stable operation of a display device is a reduction of such intrusion of a colorant across the interface. In a state in which such an image display device is operated, the two liquids may be temporarily mixed at the interface therebetween due to an effect of, for example, repeated application/non-application of voltage or unexpected shock. In the case where the state in which the two liquids have been mixed continues with the result that the interface becomes unclear and in the case where aggregates are generated at the interface, the switching performance in optical modulation brought about by a change in the size of a droplet of the colored liquid by application of voltage may be reduced or lost. Accordingly, the separated colored liquid composition also needs to have a property in which the colored liquid and the liquid immiscible with the colored liquid promptly return to the initial state of separation.
A method involving use of a surfactant may be employed to control the behavior of the colored liquid and liquid immiscible with the colored liquid at the interface therebetween in the separated colored liquid composition; however, it is known that use of typical surfactants causes problems in which a hydrophobic liquid does not have an intended dispersibility and in which the hydrophobic intermediate layer of an electrowetting device is polluted (e.g., see Patent Literature 6). From this point of view, Patent Literature 6 discloses addition of a nonionic surfactant produced through an optical ene-tiol reaction; in particular, the nonionic surfactant is composed of a highly hydrophobic compound and a nonionic and highly hydrophilic compound so as to be in the form of an emulsion ink in the production thereof and so as to be in the form of a layer in which the hydrophilic liquid has been separated from the hydrophobic liquid after the production, and the difference in the value of SP between the compound having a hydrophobic segment and the compound having a hydrophilic segment is not less than four (e.g., see Patent Literature 6).